Processing facilities and other facilities routinely include tanks for storing liquid materials and other materials. For example, storage tanks are routinely used in tank farm facilities and other storage facilities to store oil or other materials. As another example, oil tankers and other transport vessels routinely include numerous tanks storing oil or other materials.
Often times, it is necessary or desirable to measure the amount of material stored in a tank. This may be useful, for example, during custody transfer applications when material is being transferred from one party to another, such as from a seller to a buyer. During these types of applications, the amount of material in a tank often must be measured with high precision. In bulk storage tanks, an error of one millimeter in a level reading can correspond to several cubic meters of volumetric error. This can result in losses of thousands of dollars for one or more parties. High-precision measurements often require high accuracy (such as ±1 mm) over a wide range of temperatures (such as −40° F. to +185° F.).
One approach to measuring the amount of material in a tank involves the use of radar measurements. In this approach, radar signals are transmitted towards and reflected off the surface of the material in the tank. Radar accuracy is often directly associated with the stability of frequency signal generation. However, radar signals are often generated using voltage-controlled oscillators (VCOs), and voltage-controlled oscillators typically suffer from ambient temperature variations and high noise levels, particularly when used with higher-frequency electromagnetic waves such as millimeter waves (MMW). As a result, analog components and circuits often need to implement complicated compensation circuitry to cope with temperature variations and time drifts that occur during the frequency signal generation. These traditional solutions are often expensive and sometimes awkward, especially for frequencies higher than 20 GHz.
A phase-locked loop (PLL) can be used to stabilize a voltage-controlled oscillator by forming a closed loop so that a frequency produced by the voltage-controlled oscillator is relatively stable or “locked.” This solution is effective if the frequency range of the voltage-controlled oscillator can be covered by the phase-locked loop's bandwidth. This is typically true for the frequency range below 10 GHz because of limitations of current phase-locked loop chips. For frequencies higher than 10 GHz, a dielectric resonance oscillator (DRO) is often adopted as a local oscillator to down-convert higher frequencies to lower frequencies that can match a phase-locked loop's tuning range. However, dielectric resonance oscillators are still susceptible to temperature variations, which results in variations of the locked frequencies. This also introduces errors in signal processing using signal frequency and/or bandwidth information. One reason for using higher frequencies in radar level gauging technologies is that national and international regulations may limit the use of larger bandwidths at lower frequencies. These regulatory constraints can have a negative impact on high precision radar level measurements.